The present invention relates generally to cellular systems that employ a time division multiple access (TDMA) channel strategy. More precisely, the present invention relates to a load sharing method for a base station equipped with a multi-carrier power amplifier (MCPA).
A base station architecture employing a multi-carrier power amplifier (MCPA) is illustrated in FIG. 1. In FIG. 1, the base station 100 includes N transceivers 110l-110N which send and receive signals on a predetermined frequency. The transceivers 110l-110N send signals to be transmitted to the multi-carrier power amplifier (MCPA) 120 where they are amplified and linearized.
Base station 100 also includes a sensor 150 which measures the power output from the antenna 140. Controller 130 controls the power output from MCPA 120 in response to the output power measured by the sensor 150. As will be appreciated by those skilled in the art, the base station 100 includes additional circuitry which aids in the sending, receiving and processing of data.
Designing MCPAs with a high output power is a difficult and expensive task. As the MCPA is designed to have a higher maximum output power, design costs become increasingly more expensive. For a base station operating using time division multiple access (TDMA), the maximum total output power of the base station limits the total output power of the frequency carriers at any time slot. TDMA, as one skilled in the art will appreciate, is a communication technique whereby different signals are assigned to different time slots on the same frequencies. One problem commonly associated with MCPAs designed for a particular output power and operating in a TDMA environment occurs when the desired total output power for any time slot exceeds the maximum allotted power for that time slot. In such an event, the MCPA loses linearity resulting in a decrease in link quality.
The following example illustrates the above-identified problem. Consider the time chart set forth in FIG. 2. In FIG. 2, seven frequencies (1-7) in use by an exemplary base station are illustrated over eight time slots. The numbers in the time chart indicate the required output power, in watts, for a mobile unit which is operating at a particular frequency and assigned to a particular time slot. For example, at frequency 1 and time slot 1, the mobile unit requires 4 watts (W). The total power for each time slot is depicted below the time chart. For time slot 1, for example, the total output power for frequencies 1-7 is 15 W. Assume, for this example, that the MCPA has been designed such that the total maximum output power for any time slot is 30 W. Now suppose that the user that is currently using frequency number 7 and time slot number 1 wants to increase its output power from 3 W to 8 W. This increase would increase the total power for time slot 1 to 20 W which would still be within allowable limits. If, however, the user at frequency 7 and time slot 7 wanted to increase its output power from 4 W to 8 W, the total power for time slot 7 would exceed the maximum allotted output power of 30 W. In such an event, the user requesting the higher power would likely not be granted the requested increase in output power thereby resulting in a decrease in the quality of the link associated with that user.
Several techniques have been developed in order to prevent systems, such as the MCPA described above, from exceeding the maximum capacity for which they have been dimensioned. Load sharing is one such technique. Conventional load sharing is basically a type of load balancing where a user is transferred from one cell which has reached its maximum capacity to another cell which can accommodate the user. This technique avoids overload situations. The following patents illustrate conventional load sharing techniques.
A method of balancing the load among cells which are operating at maximum capacity is described in U.S. Pat. No. 4,670,899, by Brody et al., and entitled xe2x80x9cLoad Balancing for Cellular Radio Telephone Systemxe2x80x9d. In Brody et al., the loading of various cells is dynamically redistributed by selectively transferring ongoing calls to adjacent cells in accordance with traffic levels in order to reserve channels for handoffs and for new calls. A channel occupancy level for a cell is periodically determined by comparing the number of channels utilized to the number of channels available within the cell. Calls are handed off before all the channels are utilized, thereby allowing at least one or more channels to be reserved for new or incoming calls.
According to the Brody et al. patent, if there is a mobile unit on the periphery of the cell which is also within the range of a neighboring cell, the mobile unit will be transferred to the neighboring cell in order to make room for a new call or an ongoing call associated with a mobile unit which will be handed off to the cell. While Brody et al. provides traffic-based control for call handoffs from one cell to an adjacent cell, handoffs due to load balancing are handled differently from handoffs due to mobile units leaving the cell. This creates a very complex system.
In U.S. Pat. No. 5,625,868 to Jan et al., and entitled xe2x80x9cDynamic Traffic Load Distribution Methodxe2x80x9d, the control over a call is transferred from a first satellite to a second satellite having a partially overlapping coverage area with the first satellite when the power consumption level in the first satellite exceeds a certain predetermined level. This is accomplished by switching the channel off in the first satellite and on in the second satellite.
In commonly assigned U.S. Pat. No. 5,241,685 to Bodin et al., and entitled xe2x80x9cLoad Sharing Control for a Mobile Cellular Radio Systemxe2x80x9d, the entirety of which is incorporated by reference herein, a load sharing method is set forth which is based upon the occupancy of the channels defined by the ratio between the number of used occupied channels to the number of available channels.
The present invention distinguishes over the above-identified patents by providing a load sharing method which is invoked based on a power measurement of the base station""s associated MCPA. The present invention recognizes that one may want to limit the maximum output power from an MCPA in order to reduce costs. The load sharing method of the present invention enables for a lower cost, lower power MCPA to be employed in a base station.
The present invention seeks to overcome the problems of congestion and call blocking in a system employing under-dimensioned MCPAs by reallocating transmission resources in an intelligent manner such that the sum of the output power from a number of transceivers utilizing the same MCPA does not exceed the limit of the MCPA output power.
According to an exemplary embodiment of the present invention, the power of all transceivers served by a certain MCPA is compared to a threshold value X. The value X is at least related to the capability of the MCPA in terms of output power. If at no times (i.e., at all time slots), the sum of the output power exceeds the threshold value X, it is assumed that the MCPA can handle all simultaneous transmissions. If, during at least one time slot it is found that the required output power exceeds the capability of the serving MCPA, then a reallocation or load sharing algorithm is invoked. The reallocation algorithm searches for time slots to which a reallocation could be performed within the same base station or number of transceivers served by the MCPA. If no time slots can handle users from the time slot in which the MCPA limit is exceeded, then a load sharing algorithm is activated and one starts to look for transmission resources in transceivers served by other MCPAs, e.g., other cells. A number of alternative embodiments of the invention is described, which relates to the way in which the reallocation or load sharing is performed. By reallocating or moving users of a particular kind, e.g., single slot users, and avoiding reallocation of users of other particular kinds, the load sharing and reallocation are further improved.